The present invention relates to a mobile communication system in which radio connection between a radio base station and a mobile station is established by a TDMA-TDD (Time Division Multiple Access-Time Division Duplex) scheme and, more particularly, to a mobile communication system in which a given radio base station senses carrier signals intermittently transmitted from other radio base stations through control channels to detect the transmission timing of a control carrier to be transmitted from the given radio base station, and a control channel setting method in the mobile communication system.
In a conventional mobile communication system, as disclosed in Japanese Patent Laid-Open No. 7-264659, a plurality of radio base stations are connected to a public network, and radio connection between each radio base station and a mobile station is established by the TDMA-TDD scheme. If the control channels used by the respective radio base stations have the same frequency, each radio base station detects its own transmission timing on the control channel, and intermittently transmits data (carrier signals) at the detected transmission timing.
FIGS. 5A to 5E show the transmission timings of control carriers in radio base stations. Assume that radio base stations A and B are connected to a public network and start their operations to transmit carrier signals a and b on control channels at predetermined intermittent transmission periods, as shown in FIGS. 5A and 5B.
A radio base station C adjacent to the radio base stations A and B measures the reception input voltages of the carrier signals transmitted on the control channels from the radio base stations A and B to detect the control channels on which the signals are being transmitted from the radio base stations A and B. As a result, as shown in FIG. 5D, the radio base station C can obtain the transmission timings on the control carriers in the radio base stations A and B. Assume that transmission timings T0 to T3 and R0 to R3 are set as shown in FIG. 5C in the radio base station C. In this case, the transmission timings of the carrier signals a and b in the radio base stations A and B which are measured by the radio base station C are the transmission timings T1 and R0.
Upon detection of the transmission timings of the control carriers in the adjacent radio base stations A and B, the radio base station C obtains transmission allowable intervals excluding the transmission timings T1 and R0 as indicated by the hatched portions in FIG. 5E, and determines one of the transmission timings T0 to T3 and R0 to R3 in the obtained transmission allowable intervals as the transmission timing of a control carrier to be transmitted from the radio base station C, thereby preventing interference with the control channels in the radio base stations A and B.
In the above conventional system, in consideration of the transmission timings of the control carriers in the adjacent radio base stations A and B, the transmission timing of the control carrier in the radio base station C is set to prevent contention with the transmission timings under consideration. For this reason, the reception timing (reception slot) of a control carrier from a mobile station which exits in the zone covered by the radio base station C and demands connection may contend with the transmission timings (transmission slots) in the radio base stations A and B. In this case, the radio base station C cannot be connected to the mobile station in the zone, resulting in a deterioration in connectivity.